It is known that phenols can be hydrogenated catalytically to cyclic ketones. U.S. Pat. No. 2,829,166 describes the hydrogenation of phenols in the presence of palladium catalysts to give the corresponding cyclohexanones. A method for hydrogenating p-tert-amylphenol is known from DE 29 09 780 A1. A method for preparing substituted cyclohexanones is disclosed in EP 731 075 A1. Furthermore, EP 889 019 A1 and EP 890 565 A1 disclose methods for preparing cyclohexanones by hydrogenation of the corresponding phenols in the presence of alkanes or water as solvent. The use of aliphatic alcohols as solvent is not disclosed in these documents.
Good selectivities for cyclohexanones are generally obtained at temperatures of 120-250° C. and a hydrogen pressure of 1-20 bar. In this case it may be advantageous to treat the Pd/C catalysts with an alkali metal carbonate or alkaline earth metal carbonate or other basic or neutral salts or to add basic or neutral salts to the hydrogenation mixture.
Solvents which can be used for the hydrogenation of phenols are however limited. Hydrogenation in the absence of solvent is described (cf. U.S. Pat. No. 2,829,166), however this is only feasible for phenols which melt in the range of the hydrogenation temperatures. The only other known solvents are ethers (cf. EP 731 075 A1), specific alkanes such as, for example, methylcyclohexane (cf. EP 889 019 A1) or ethers (cf. EP 890 565 A1). The catalytic hydrogenation of phenols in aromatic compounds as solvents, such as toluene or xylene for example, is further described by M. Wydra and H. Vogel in Chemie Ing. Techn. 74, 800-804, 2002.
A method for preparing hydroxycyclohexanones is known from JP 11-060534 A, in which substituted polyphenols, such as resorcinol, hydroquinone or pyrogallol, are hydrogenated in the presence of a palladium catalyst comprising alkali metal, in a saturated monohydric C3—C12-alcohol. The Pd catalyst may be generated for example by treatment with alkali metal hydroxides.
It is known that, when using relatively polar solvents such as, for example, alcohols, which would clearly broaden the range of application of this reaction, substantially lower turnovers and selectivities occur. For example, Tagaki et al. in Energy & Fuels 13 (6), 1191-1196, 1999 describe the ring hydrogenation of various aromatic compounds and found that, in the case of catalysis by Pt/C, the reactivity was greatly reduced by alcohols. Higashijima and Nishimura in Bull. Chem. Soc. Jpn. 65, 2955-2959, 1992 describe investigations relating to the ring hydrogenation of phenol and cresols over Pd/C catalysts in various solvents and found that in alcohols both the hydrogenation rate as well as the selectivity in favour of the cyclohexanones, had been considerably reduced in comparison to the hydrogenation in alkanes or without solvent. This effect has been found when using commerically available Pd/C catalysts for diverse primary, secondary and tertiary alcohols with 3 or more carbon atoms; the effect was somewhat weakened for acid treated Pd/C catalysts.
A method for the hydrogenation of phenols is disclosed in U.S. Pat. No. 4,409,401, in which a palladium catalyst specifically poisoned with sulfur is used. The preparation of the catalyst takes place by oxidative and subsequently a reductive treatment. This document states that during the hydrogenation it is advantageous for basic substances to be present, for example Na carbonate. The reaction can take place in the presence of a diluent. Examples of the latter are saturated and aromatic hydrocarbons, alcohols or the desired product of the method.
EP-A-822,173 discloses a method for preparing 1,3-cyclohexanediones. Starting from 1,3-bisphenols, a transfer hydrogenation method is carried out, in which a hydrogen donor, typically a secondary alcohol or formic acid or one of its salts, hydrogenates the starting material in the presence of a palladium catalyst. A classical hydrogenation by means of hydrogen is not disclosed.
US-A-2010/041714 relates to the preparation of bisarylaminotetralins. In one working example the preparation of a precursor of 5-hydroxy-3,4-dihydro-2H-naphthalen-1-one is described. This takes place by hydrogenation of 1,5-dihydroxynaphthalene in isopropanol and aqueous sodium hydroxide in the presence of a palladium catalyst supported on carbon. The proportions of palladium and of aqueous sodium hydroxide, based on the bisphenol used, are quite high in this example.